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Bob and others,
This technique of using a field winding in series with the
armature to counteract the main field winding for speed stabilization is rather
old. I have an electrical machinery book from around 1974/75 that
describes this method. This book desribed several DC motor control schemes
from when solid state controls were unheard of.
To accomplish the same thing today they use a technique
called IR compensation, but that requires a slightly more complicated control
design, although it still can be analog. Typically if you load a DC motor,
that is without these compensation windings in series with the armature,
the current goes up and the speed goes down. The "theory" being that as
the current goes up more of the drive's output voltage is consumed overcoming
winding resistance, an IR drop, and less is available to get the motor to
speed. (A theoreticaly perfect DC motor with no resistance would have
speed exactly proportional to armature voltage and torque exactly proportional
to armature current when the field strength is fixed.) What happens with IR
compensation properly adjusted is that when motor current goes up the output
voltage of the DC drive is boosted so that the IR drop is exactly compensated
for and the voltage needed to keep the motor at speed remains constant even
under this changing load.
You can also set IR compensation so that a DC motor will
speed up under load, or slow down. That is one reason why in an E15 and
E20 they don't want field weakening in reverse. The field compensation
winding in series with the traction motor armature that weakens the field when
going forward strengthens the field in reverse.
Steve Naugler
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